Gerardi Monize, Ramírez-Hernández Alejandro, Binder Lina C, Krawczak Felipe S, Gregori Fábio, Labruna Marcelo B
Department of Preventive Veterinary Medicine and Animal Health, Faculty of Veterinary Medicine, University of São Paulo, São Paulo, Brazil.
Front Physiol. 2019 May 28;10:653. doi: 10.3389/fphys.2019.00653. eCollection 2019.
The bacterium is the etiological agent of Brazilian spotted fever (BSF), which is transmitted in Brazil mainly by the tick Herein, larvae and nymphs of six populations of were exposed to by feeding on needle-inoculated guinea pigs, and thereafter reared on uninfected guinea pigs or rabbits. Two tick populations were exposed to autochthone strains, whereas four tick populations were exposed to non-autochthone strains. The six geographically different populations of showed different susceptibilities to , higher among the two tick populations that were exposed to their autochthone strain. In addition, higher rates of transovarial transmission of and vector competence success also included the two tick populations that were exposed to autochthone strains. These results indicate that the susceptibility of to varies among different tick populations, with a clear bias for higher susceptibility to an autochthone strain that has already coevolved with a tick population for some time. Our results demonstrated that the infection induces higher mortality of engorged larvae and nymphs, and tend to reduce the reproductive fitness of engorged females. All together, these results might explain the low infection rates of under natural conditions (usually <1%), and indicate that an population should not be able to sustain a infection for successive tick generations without the creation of new cohorts of infected ticks via horizontal transmission on vertebrate rickettsemic hosts (amplifying hosts). Finally, despite of the ubiquitous distribution of in southeastern and central-western Brazil, most of the populations of this tick species are devoid of infection. This scenario might be related to two major factors: (i) insufficient numbers of susceptible amplifying hosts; and (ii) lower susceptibilities of many tick populations. While the first factor has been demonstrated by mathematical models in previous studies, the second is highlighted by the results observed in the present study.
该细菌是巴西斑疹热（BSF）的病原体，在巴西主要通过蜱传播。在此，六个种群的蜱幼虫和若虫通过吸食经针刺接种立克次氏体的豚鼠而接触立克次氏体，然后在未感染的豚鼠或兔子身上饲养。两个蜱种群接触本地立克次氏体菌株，而四个蜱种群接触非本地菌株。六个地理上不同的蜱种群对立克次氏体表现出不同的易感性，在接触其本地立克次氏体菌株的两个蜱种群中易感性更高。此外，立克次氏体的经卵传播率和媒介能力成功率较高的情况也出现在接触本地立克次氏体菌株的两个蜱种群中。这些结果表明，蜱对立克次氏体的易感性在不同蜱种群中有所不同，明显倾向于对已经与蜱种群共同进化一段时间的本地立克次氏体菌株有更高的易感性。我们的结果表明，立克次氏体感染会导致饱血幼虫和若虫的死亡率更高，并倾向于降低饱血雌蜱的生殖适应性。总之，这些结果可能解释了在自然条件下立克次氏体的低感染率（通常<1%），并表明在没有通过脊椎动物立克次体血症宿主（扩增宿主）的水平传播产生新的感染蜱群的情况下，蜱种群应该无法在连续几代蜱中维持立克次氏体感染。最后，尽管这种蜱在巴西东南部和中西部广泛分布，但该蜱种的大多数种群都没有立克次氏体感染。这种情况可能与两个主要因素有关：（i）易感扩增宿主数量不足；（ii）许多蜱种群的易感性较低。虽然第一个因素已在先前的研究中通过数学模型得到证明，但第二个因素在本研究中观察到的结果中得到了突出体现。
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